Sensors are elements that permit the measurement of physical phenomena that are conventionally designated as size or parameters. They translate the state or evolution of these sizes or parameters in the form of electrical signals. They thus provide information on the development of and, subsequently, the static and dynamic behaviors of the processes in which they are implemented.
Sensors are currently widely available and are used in virtually all sectors of activity. They are used to measure temperature, pressure, position and level to cite only the most important. Studies and catalogs prepared, by specialists set out their characteristics as well as their uses. By virtue of these catalogs, it is possible to know the magnitude of the electrical signals corresponding to the physical size or parameter being determined. The term of xe2x80x9cconversion scalexe2x80x9d is used to mean that there exists a coherent relationship between the physical size and the electrical signal. For example, if one considers a pressure sensor, an electrical signal magnitude in the form of a voltage or a current will correspond to the pressure magnitude of the sensor.
Current physics, electrical and electronic technologies allow to conceive and build a sensor. The ease of fabrication and use of a sensor will determine its cost, the areas in which it will be used and, in consequence, its market.
Certain sensors are not as easily designed for reasons that are linked to the nature of and the manner in which a physical parameter is available. For sensors intended to measure forces, even if it is, in principle, easy to measure a force which develops in a metal part, the capture of this force, economically, is not so simple.
The measure of forces that are applied to a metal part usually takes into account the deformation of the material submitted to tensile strain, compression, torsion strain and/or the combination of the action of these forces.
The size of the deformation as a function of the forces acting thereon determines the gradient, that it to say the precision by which one can evaluate the elementary distortion. It is therefore essential to know the relationship between such deformations in microns or millimeters and the forces applied on the part.
The gradient is determined by a mathematical study of the resistance of the materials used, to which is correlated, according to the drawing of the part, a simulation of the finite elements in order to establish the relationship between the deformations and of the forces.
It is readily apparent that it is not easy to know the totality of the gradients because of the diversity of the materials and the shapes of the parts. Each part is a particular instance and, to use a sensor destined to measure forces or deformations, a certain number of rules must be respected. The exact locations where the forces develop and what are the maximum and minimum deformations that are produced by the result of these forces must be precisely known.
Once these elements are known, a few components and techniques allow to obtain an electrical signal which is representative of the deformations which develop in the part. These components are constraint gauges, piezo resistive elements, capacitive elements, optical devices that allow to measure the change in granulometric orientation of materials or ferro-magnetic components whose displacement in a magnetic field determines an electrical signal in relation with the deformation of the part.
Currently, even if these components are known and regularly implemented, their setting up on a mechanical member is not easy and the repetitivity of the value of the information is not very constant when one adds or replaces a sensor by another. It is necessary, in most cases, to recalibrate the sensor and, in doing so to, adjust the zero and the scale of the electrical signals.
The sensors destined to measure forces are usually implemented according to two methods.
The first method consists of sticking a bridge of constraint gauges to the location of the deformation. In this case, the value of the signal is closely related to the precision by which the bridge of gauges is positioned and oriented on the part, and to the uniformity of the pressure with which the bridge of gauges is applied to the part. It is not possible to precisely position the bridge of gauges mechanically with respect to the development location of the forces on the piece because of its own nature. Since the signal of the bridge of resistive gauges is, for example, determined by the equation R=p L/S wherein R is the value of the resistance which evolves in function of the elongation and of the section of the conductor which forms the bridge of gauges, p is the resistivity of the materials forming the resistor, L is the length of the conductor and S is its section, the values of L and S may be affected by the method used to position the bridge of gauges. This is true with bridges, whatever technology: piezo resistive, capacitive or other, when the gauges are positioned directly on pieces for which the mechanical state is rough and the dimensions are average or important.
The second method consists of sticking a bridge of gauges on a metal blade whose characteristics are known as described in the patent application Ser. No. CA 2,198,537 filed Feb. 26, 1997, and thus ensuring a repetitivity in the relationship between the mechanical deformation of the and the variation of the resistance of the element of the bridge if a resistive bridge is used for example and then mounting the blade on a mechanical member on which forces develop. The problem in this case is that the materials of the blade are not necessarily of the same nature than the materials that form the mechanical devices and that the adaptation of the assembly of the blade with the mechanical members is difficult when setting up the sensor in the mechanical environment while keeping a constant relationship between the physical size and the electrical signal calibrated during fabrication. It is thus necessary to take into account the installation of the sensor to optimize the scale of conversion of the mechanical forces into electrical signals.
U.S. Pat. No. 5,522,270 by Gissinger et al. describes a device to measure the stress exerted on a mechanical member and a method to install this device. A gauge generates an electrical current in accordance to the stress exerted on the part. The gauge is pre-bent in plant as illustrated in FIG. 5 with four tension points located at the folds. The problem that person skilled in the art have identified with this device is that the points of tension make the buckling a lot less precise. Furthermore, the repetitivity of the buckling is difficult as each blade reacts differently because of the tension points. By stretching the Gissinger et al. gauge by points A and B, the blade can buckle but the settings will not be very precise since the forces are modified by the folds. Stretching the blade of Gissinger et al. By points A and B does not permit a linear deformation of the blade. Furthermore, the suggested fastening methods cause a total embedment of the ends. They are susceptible of causing in the blade and in the folding rays very high constraints. They also make the blade very sensitive to undesirable constraints such as deformations due to installation and thermal constraints generated by the temperature variations of the part to be measured. These constraints can induce a signal highly superior to the size to measure. It is necessary to find a configuration of the blade that will allow a repetitivity of the buckling and the construction of the device.
Taking into consideration these constraints leads to trying to eliminate the disadvantages previously cited and to make the relationship between the mechanical piece and the sensor as accurate as possible.
The present invention considers that tensile testing is the way to measure forces. It describes the processes, which will allow implantation of a sensor in a device to be measured and will ensure repetitivity.
Repetitivity is the relationship between the deformation of the sensor to which it is subjected and the electrical signal it delivers. This repetitivity imposes that the relationship of the deformation of the receiving piece to be measured is in correspondence with the deformation of the internal blade of the sensor and that the initial jamming of the sensor and the adjustment to its scale of conversion be thus compatible before and after assembly on the receiving piece.
For example, MultiDyn commercializes a sensor equipped with two bearing parts, a blade on which is mounted a bridge of gauges as described in existing literature. During the mounting of the sensor with the mechanical members, the deformations that are produced in the sensor are transmitted to the bearing parts of the blade which cause a tensile or a compressive strain of the blade. This is translated into the bridge of gauges by a variation of the resistance of the components that make up the branches of the bridge and consequently of the electrical signal at the terminals of the bridge. The calibration curve of the sensor established during fabrication is thus falsified, which forces a carrying out of a new calibration on site, which does not guarantee the accuracy of the relationship between physical size and electrical signal and increases the time and cost of assembly.
This invention can advantageously replace all of these types of tensile sensors and can be used as a sensor for stress, torque, and in certain cases, as a sensor for displacement.
Here are thus the main problems of the existing sensors that this invention can replace. Many of these tensile sensors are adapted to certain types of use but all have disadvantages. The resistive gauges glued or welded directly on the structure to be measured are devices which are very reliable and accurate. However, their direct implantation requires laboratory work and is not possible without taking great and costly precautions, that is why, for common measurements, they are generally coupled to a test body easier to implement. The methods of external observation of the structure, with or without preparation of the surface, such as the photoelastometry, moire, crackling polish, holography, X diffraction, are rather used as laboratory techniques of laboratory constraint analysis.
The test bodies equipped with gauges replacing a part of the structure, or inserted at the points of transmission of stress, are very costly, hard to install, and often provoke a weakening of the structure. The test bodies coupled to these structures generally show high stiffness, the stress at the abutments is very high and the influence on the structure may be important. The sliding at the abutments is hard to avoid, provoking important errors of return to zero setting and of hysteresis. The mechanical tensile sensors are delicate to install and are fragile and costly. The piezo-electrical sensors only work in compression, require high pressure and a complicated assembly. Furthermore, they only take into account the dynamic signals. And finally, piezo-resistive sensors are very fragile (silicone cells) and derive enormously in temperature.
The aim of the invention is to bring a solution to the problem described above by maintaining the bearing parts of the blade stationary between the moment of the calibration of the sensor on a calibration bench and the mounting on the mechanical member.
The invention consists in adjusting the offset and the gain by mechanically varying the dimension between the axes of the bearing parts, then, once the adjustment of the linearity of the curve is obtained, to adjust the offset for a value of the electrical signal corresponding to a known dimension and, then, to lock the degrees of freedom by a permanent mechanical device with the help of a flange. The flange is made of a film of metallic material, composite or polymer, and is placed between the bearing parts. It keeps the adjustment of the calibration made on the bench until the mounting of the sensor on the mechanical member by known classical industrial processes of bonding, welding or other. When the sensor and the mechanical member are coupled, and thus strictly bound to one another, one proceeds with the rupture of the flange and this ensure a amming without further adjustment at mounting. The rupture of the flange allows to free the functioning of the sensor.
The improvement of the device with respect to the other existing devices up to date is the ease of installation and the reduced cost while keeping excellent precision and reliability. Furthermore, it is robust, mobile, reusable and not very sensitive to mechanical disturbances.
Another embodiment of the invention comprises a tensile testing sensor that includes two abutments capable of being fastened jointly to a mechanical member to be measured, an elastic blade isostatically supported at its ends between the two abutments upon which the elastically blade is kept in flexion by buckling, means to measure the deformation of the elastic blade resulting from the displacement of the abutments in order to determine the section in the mechanical member.